1. Field of the Invention
This invention relates to a pigment dispersion which is suitable for the preparation of coating compounds and contains at least one solvent and a pigment dispersing agent, which pigment dispersing agent comprises a copolymer containing OH (hydroxy) groups and containing ethylenically unsaturated monomers incorporated by polymerization. The invention also relates to the use of such pigment dispersing agents for the preparation of coating compounds, in particular lacquers.
2. Description of the Related Art
Coating compounds such as lacquers, paints, fillers and primers contain film forming binders and pigments as their main constituents as well as the usual additives. One major problem in the preparation of such coating compounds is that of achieving satisfactory dispersion of the pigments and at the same time ensuring that an optimum state of dispersion, once obtained, is not lost during storage of intermediate products used for the preparation of lacquers or of the lacquers themselves, or during working up and application of the products. The said intermediate products are mainly pigment pastes and grinding pastes. Pigment pastes containing the desired pigments and pigment dispersing agents as well as other additives in solvents are available on the market. Grinding pastes frequently also contain binders of the kind used in the coating compound into which the pastes are to be made up. These problems are particularly difficult in lacquers used for motor car repairs because these are sold in relatively small quantities and often stored for a very long time in the repair works of garages before they are used.
The quality of a pigment dispersion also depends on the nature of the binder in the coating compound and the nature of the pigment. Coating compounds containing, for example, alkyl resins as binders generally provide more stable pigment dispersions than, for example, acrylic resins. Some pigments are readily dispersed and form stable dispersions while others, known in the art as "critical" pigments, give rise to considerable problems; they are very difficult to disperse and keep in dispersion and the coating compounds containing them therefore have a poor storage stability.
Some pigments, including some of those described in the Examples, tend to flocculate and float to the surface if they are not well dispersed and may also cause other pigments present to undergo this form of separation. Coarsening of the pigment particles and floating to the surface frequently affect the intensity and gloss of the color and its position in the spectrum (shift of the optical remission of the finished paint to other wave lengths).
When the known dispersing agents of the prior art are used for the preparation of coating compounds, flow problems may arise, e.g. the substances may be insufficiently fluid in the mills (e.g. in closed ball mills with stirrers). This adversely affects the energy consumption and the output.
This problem has partly been solved to a more or less satisfactory extent by the use of so called pigment dispersing agents as described, for example, in DE-C-22 18 613, EP-A-52 224 and EP-A-154 678 and by J. Toole in Developments in Hyperdispersants Technology for Paints: Paint & Resin--February 1985. Experiments have shown that when the pigment dispersing agent used in EP-A-52 224, Example 1, is used for very finely divided, high quality pigments for motor car top coat lacquers, they give rise to very poor viscosity characteristics so that the pigments are virtually impossible to use in ball mills with stirrers. The bone soot mentioned in Example 1 of EP-A-52 224 is not one of the finest high quality pigments. The figures given for the fineness of grinding in EP-A-52 224 also indicate incomplete dispersion or the use of less than high quality pigments which can be dispersed with a relatively small quantity of dispersing agent.
The pigment dispersions described in the Examples of DE-C-22 18 613 contain very high proportions of dispersing agent, based on the quantity of pigments. Such a high proportion of dispersing agent limits the field of application of such pigment dispersions so that they are not universally unusable.
Excellent results can in many cases be obtained with the dispersing agents described in EP-A-154 678 although the problems described above may occur in some of the particularly critical pigments. The following are examples of such very critical pigments:
quinacridone C.I.: P.R. 122 PA1 red azo pigment C.I.: P.R. 170 PA1 tetrachlorothioindigo C.I.: P.R. 88 PA1 perylene C.I.: P.R. 179 PA1 tetrachloro-isoindolinone C.I.: P.Y. 110 PA1 highly oxidized carbon black C.I.: P.B1. 7 PA1 phthalocyanine blue C.I.: P.B. 15:4 PA1 chlorinated and brominated phthalocyanines C.I.: P.G. 7 and P.G. 36. PA1 (a) 0.5 to 40% by weight of N,N-di-C.sub.1 to 4-alkylamino-C.sub.1 to 8-alkyl(meth)acrylamides and /or a mixture of N,N-di-C.sub.1 to 4-alkylamino-C.sub.1 to 8-alkyl(meth)acrylates and N- substituted (meth)acrylamides and/or (meth)acrylamide, wherein the ratio of amino(meth)acrylates to amido(meth) acrylates is preferably in the range of from 1:2 to 2:1, PA1 (b) 10 to 40% by weight of hydroxy-C.sub.2 -8-alkyl(meth)acrylates, preferably hydroxy-C.sub.2 -4-alkyl (meth) acrylates and PA1 (c) 20 to 89.5% by weight of copolymerizable .alpha., .beta.-olefinically unsaturated compounds; with